Changed `source` to `sources` in source map, added coffee file in that list

These are the modifications I had to do in order to get source maps working
in 27.0.1425.2 (Official Build 185250) canary. I haven't tested other
browsers.

After doing the `source` to `sources` change, the coffee source and for
some odd reason the javascript file would not show up in the dev tools
(it was being fetched by the browser, but not evaluated).

To fix this, I had to add the coffee source to the `sources` list in the
source map file.